1. Field of the Invention
The present invention relates to an optical pickup apparatus capable of reading two or more kinds of recording media of different read wavelengths, such as an optical pickup apparatus compatible for DVD (Digital Versatile Disc) and CD (Compact Disc), and, more particularly, to an optical pickup apparatus that uses a semiconductor laser device constituting a one-chip laser diode which emits two laser beams of different wavelengths.
2. Description of Related Art
There have been active proposals of DVD/CD compatible players which use both the optical pickups of a CD player and a DVD player. Those players are classified into a DVD/CD compatible player using a one-wavelength bifocal optical pickup and a DVD/CD compatible player using a two-wavelength bifocal optical pickup.
A description will now be given of the comparison of the structures of a CD and DVD with each other. The thickness of the protection layer of a DVD (which is 0.6 mm) is about a half of that of the protection layer of a CD. In the case where both optical disks are played using a unifocal optical pickup, when focused on the information recording surface of a CD in the way that is optimal for the information recording surface of a DVD, the light beam will have an aberration, such as spherical aberration, because the protection layer of the CD that the light beam passes is thicker than that of the DVD. This disables the optimal light focusing on the information recording surface of the CD. As the size of information pits formed to record information differs between the CD and DVD, a beam spot of the optimal size for information pits on the CD and a beam spot of the optimal size for information pits on the DVD should be formed respectively on the information recording surfaces of the CD and DVD in order to accurately read the respective information pits.
The size of a beam spot is proportional to the ratio of the wavelength of a laser beam to the numerical aperture of an objective lens which condenses the laser beam onto the information recording surface. That is, given that the wavelength of the laser beam is constant, the greater the numerical aperture is, the smaller the beam spot becomes. In the case where both a CD and DVD are played using a unifocal optical pickup, therefore, if the wavelength of the laser beam is designed constant and the numerical aperture of the objective lens is designed adequate for the information pits of, for example, the DVD, the beam spot would become too small for the information pits of the CD. This produces distortion in signals reproduced from the CD, thus making it hard to read those signals accurately. To cope with this situation, the current trend is toward a DVD/CD compatible player that uses a bifocal optical pickup which can have focus at different positions on the same line and can form a beam spot of the adequate size for the size of each type of information pits.
The optical pickup apparatus of the DVD/CD compatible player that requires two light sources needs a synthesizing prism which makes the cost of this apparatus higher than the optical pickup apparatus that uses a single light source. In addition, if the first light source emits a light beam to one surface of the first beam splitter, the second light source should emit a light beam to the surface of the first beam splitter that is at the right angles to the first light source, requiring larger installing space for the optical systems. This inevitably enlarges the optical pickup apparatus.
Accordingly, it is an object of the present invention to provide an optical pickup apparatus which is compatible for two wavelengths without using a synthesizing prism and can thus be made compact.
To achieve the above object, according to one aspect of this invention, there is provided an optical pickup apparatus capable of reading information from recording media of different read wavelengths, which comprises light-emitting means having a first light source for emitting a first laser beam integrated with a second light source, located close to the first light source, for emitting a second laser beam of a different wavelength from that of the first laser beam; photodetection means for receiving a beam reflected by each of the recording media, the photodetection means including a nine-divided detector having nine light-receiving areas arranged in three columns by three rows; and an optical system having a plurality of optical elements which form an optical path to guide the first and second laser beams to the recording media and form an optical path for guiding the reflected beam to the photodetection means and through which the first and second laser beams pass.
In this optical pickup apparatus, it is preferable that each of the first and second laser beams should be received at four of the nine light-receiving areas of the nine-divided detector, which include a center one of the nine light-receiving areas and are arranged in two columns by two rows, and the four light-receiving areas of the nine-divided detector which receive the first laser beam should be different from the four light-receiving areas of the nine-divided detector which receive the second laser beam.
In the above preferable mode of the optical pickup apparatus, it is further preferable that at least a read signal and a focus error signal should be generated by performing an arithmetic operation on amounts of light respectively received at the four light-receiving areas.
In the first preferable mode or the second preferable mode of the optical pickup apparatus, it is further preferable that a pair of light-receiving sections to be used in generating a tracking error signal should be respectively formed on both sides of the nine-divided detector in such a range as to be able to receive both a pair of first sub beams generated on both sides of the first laser beam with the first laser beam in between and a pair of second sub beams generated on both sides of the second laser beam with the second laser beam in between.